low flame on water heater?

have a Mersey Super water heater with two problems..First prob is low flame on pilot which needs to be relit every time hot water is needed.Second prob is that the heater only fires up fully when bath and sink hot taps are run together.From what i have read on this (great and helpful ) site the first prob could be thermocouple.Could this be an expensive problem to resolve as i dont want to tackle it myself.Thanks for any input.